Inverters have been installed in many types of electric equipment, as an efficient variable-speed control unit. Inverters are switched at a frequency of several kHz to tens of kHz, to cause a surge voltage at every pulse thereof. Inverter surge is a phenomenon in which reflection occurs at a breakpoint of impedance, for example, at a starting end, a termination end, or the like of a connected wire in the propagation system, and as a result, a voltage up to twice as high as the inverter output voltage is applied. In particular, an output pulse occurred due to a high-speed switching device, such as an IGBT (Insulated Gate Bipolar Transistor), is high in steep voltage rise. Accordingly, even if a connection cable is short, the surge voltage is high, and further voltage decay due to the connection cable is low. As a result, a voltage almost twice as high as the inverter output voltage occurs.
As coils for electric equipment such as inverter-related equipment, for example, high-speed switching devices, inverter motors and transformers, insulated wires, which are enameled wires, are mainly used as magnet wires in the coils. Accordingly, as described above, a voltage nearly twice as high as the inverter output voltage is applied in the inverter-related equipment. Then, it has been required in the insulated wires to minimize partial discharge deterioration, which is attributable to inverter surge.
In general, partial discharge deterioration means a phenomenon in which the following deteriorations of the electric insulating material occur in a complicated manner: molecular chain breakage deterioration caused by collision with charged particles that have been generated by partial discharge; sputtering deterioration; thermal fusion or thermal decomposition deterioration caused by local temperature rise; or chemical deterioration caused by ozone generated due to discharge, and the like. Due to the phenomenon, the electric insulating materials which actually have been deteriorated by partial discharge show reduction in the thickness.
It has been thought that inverter surge deterioration of an insulated wire also proceeds by the same mechanism as in the case of general partial discharge deterioration. Namely, partial discharge occurs in the insulated wire due to the surge voltage with a high peak value, which is occurred at the inverter, and the coating of the insulated wire causes partial discharge deterioration as a result of the partial discharge; in other words, high-frequency partial discharge deterioration.
In order to prevent deterioration of the insulated wire due to such partial discharge, a study on the insulated wire exhibiting a high-inception voltage of the partial discharge have been conducted. In order to obtain the foregoing insulated wire, there is a method of thickening an insulating layer of the insulated wire.
Further, aside from enhancing an inception voltage of the partial discharge by a coated resin layer provided on the outside of the enamel wire, an attempt to seek high-value-added properties by using a newly created coated resin layer has been conducted. For example, Patent Literatures 1 and 2 or the like propose to set an extrusion-coated resin layer on an enamel-baked layer.
On the other hand, in a rotary electric machine such as a motor, for storage of a coil obtained by subjecting an insulated wire to a winding work, in order to improve the proportion (occupancy) of the conductor of the coil to the volume space of the slot for storage, in consideration of both a fluidity of the resin varnish and a surface tension, Patent Literature 3 proposes to set a thermoplastic coated resin layer each of which side has an excurved shape as an outermost layer, on a rectangular conductor.